1. Field of the Invention
The present invention relates to a filter element, and more particularly relates to a distributed constant filter.
2. Description of Related Art
Cellular telephones, radio-Local Area Networks (radio-LAN), and other high frequency communication devices that use a microwave band or milliwave band carrier typically have filter elements, such as low pass filter (LPF) and band pass filter (BPF). The filter elements may be designed using a distributed constant filter formed with a conventional microstrip transmission line. Unlike filter elements that have a composite component consisting of an inductor (L) and a capacitor (C) that are combined to form an L-C circuit having a concentrated or lumped constant L-C parameter, a conventional microstrip transmission line has serially distributed L and C parts formed on a substrate as shown in FIGS. 1 and 2.
FIG. 1 is a plan view illustrating the structure of a conventional filter element 10 formed with a microstrip transmission line. The conventional filter element 10 includes a dielectric (or insulating) substrate 12 such as a ceramic substrate or a printed substrate (e.g., a dielectric, such as silicon dioxide or silicon nitride, is deposited on a substrate and then masked using known fabrication techniques to form a printed dielectric pattern on the substrate). The conventional filter element 10 also includes a strip conductor pattern 14 formed on the dielectric substrate 12, and I/O electrodes 16 and 18 that are electrically connected to the strip conductor pattern 14. The strip conductor pattern 14 includes a first group of segments 20, 22, 24, and 26 that function as inductors and a second group of segments 28, 30, and 32 that function as capacitors. Each inductor segment 20, 22, 24 and 26 has a width (e.g., width 23 of inductor segment 20 as shown in FIG. 1) of about 0.1 millimeters (mm) and a length (e.g., length 21 of inductor segment 20) of about 0.3 mm. Each capacitor segment 28, 30, and 32 has a width (e.g., width 29 of capacitor segment 28 as shown in FIG. 1)of about 5 mm and a length (e.g., length 27 of capacitor segment 28)of about 3 mm. The conventional filter element 10 shown in FIG. 1 is a microstrip line LPF that has an impedance which is varied alternately as a result of forming the strip conductor pattern 14 on the dielectric substrate 12. By forming the strip conductor pattern 14 to have inductor segments and capacitor segments that are optimally sized, a signal in a bandwidth higher than a desired frequency can be attenuated.
An equivalent electrical circuit representation 50 of the conventional filter element 10 is shown in FIG. 2. The inductor segments 20, 22, 24, and 26 correspond to the inductors 52, 54, 56, and 58, respectively. The capacitor segments 28, 30, and 32 correspond to the capacitors 60, 62, and 64, respectively. Because the inductor segments and the capacitor segments in the conventional filter element 10 have a flat structure, the filter element 10 can be formed simultaneously in a process for forming a wiring pattern on a mounting substrate using known printing or lithography techniques.
However, in forming the conventional filter element 10 as described above, a problem arises where the inductance effect (e.g., ability to oppose any change to a electrical current flowing through the filter element) of the equivalent electrical circuit 50 shown in FIG. 9 is reduced due to a parasitic capacitance of the portion of the dielectric between the substrate 12 and the strip conductor pattern 14 that occurs when a signal in the frequency range of microwave and milliwave is transmitted through the filter element 10. Parasitic capacitance, for example, may be the capacitance or collection of charge between a conduction layer, such as the strip conductor pattern 14 and a base, such as the substrate 12. Parasitic capacitance, which degrades the performance of a circuit on a substrate or chip, is not intentionally designed into the chip or circuit but is rather a consequence of the layout of the circuit on the chip. This problem of parasitic capacitance is particularly prevalent when the transmitted signal through the conventional filter element 10 is in the frequency range exceeding 5 GHz.
To prevent the reduction in the inductance effect of the equivalent electrical circuit 50 and to obtain the desired filter performance, the inductance in the conventional filter element 10 is increased by thinning the width of the inductor segments 20, 22, 24 and 26 in the strip conductor pattern 14 shown in FIG. 1. Further, to reduce the passband loss of the filter element 10, the length of each inductor segment 20, 22, 24, and 26 is reduced substantially. Passband loss, defined in decibels (dB), describes the absolute loss across a band of frequencies the conventional filter element 10 is supposed to pass.
By substantially reducing the width and the length of the inductor segments 20, 22, 24, and 26 within the strip conductor pattern 14, the resulting conventional filter element 10 has the following other problems:
1) The inductor segments 20, 22, 24, and 26 may require micrometer (xcexcm) order accuracy in fabrication, making it difficult to obtain a high production yield for the conventional filter element 10.
2) The significantly reduced length of the inductor segments 20, 22, 24, and 26 results in an unintentional strong electromagnetic coupling between respective adjacent capacitor segments 28, 30, and 32, which impacts the desired performance of the filter element 10.
3) The difference in line width between the inductor segments 20, 22, 24, and 26 and the capacitor segments 28, 30, and 32 is significantly large. The line width of one capacitor segment (i.e., 28, 30, or 32 in FIG. 1) may be 10 times that of the one inductor segment (i.e., 20, 22, 24, and 26 in FIG. 1). The large difference in line width causes a large stress at the contact or connection between the inductor segments 20, 22, 24, and 26 and the capacitor segments 28, 30, and 32 as a result of temperature cycling during operation of the conventional filter element 10. The large stress may cause a disconnection between a respective inductor segment and capacitor segment in the strip conductor pattern 14. Thus, the conventional filter element 10 has poor reliability due to this disconnection problem.
4) If a device which generates heat during operation, such as a power amplifier, is mounted on the substrate 12 on which the filter element 10 has been formed, heat from the power amplifier may burn or melt one of the thin inductor segments 20, 22, 24, and 26, causing a disconnection in the strip conductor pattern 14.
Thus, a filter element that is formed with a conventional microstrip line has several significant problems, such as low production yields due to the difference in size in line width of the inductor segments and capacitor segments formed in the conventional microstrip line, and disconnections in the conventional microstrip line due to the stress caused between connections of inductor segments and capacitor segments during temperature cycles of the conventional microstrip line.
The present invention works toward providing an improved filter element that is formed with a microstrip line that has uniform line width to effectively improve the production yield and reliability of the improved filter element. The present invention also works toward providing a fabrication method for producing the improved filter element at high production yield.
The present invention provides a filter element fabricated by forming a strip conductive pattern on a dielectric substrate that has a surface and a cavity with an aperture disposed on the surface of the dielectric substrate, wherein the strip conductive pattern is formed over the aperture of the cavity.
The present invention also provides a filter element fabricated by forming a strip conductive pattern on a dielectric substrate that has a first portion and a second portion, the first portion having a higher relative dielectric constant than the second portion, wherein the width of the strip conductive pattern is maintained constant and the strip conductive pattern is formed over both the first and second portions of the dielectric substrate.
The present invention provides a method for fabricating a filter element that includes a strip conductive pattern on a dielectric substrate, wherein the method for fabricating the filter element comprises forming a cavity with an aperture disposed on the surface of the dielectric substrate, filling a material in the cavity so as to flatten the surface of the dielectric substrate, forming the strip conductive circuit pattern on the dielectric substrate so that the strip conductive pattern is over the aperture of the cavity, and removing the material from the cavity.